1. Field of the Invention
The present invention relates to a focus search method for finding a position of an objective lens disposed on a pickup that is a unit for irradiating light to a recording medium, by moving the pickup, wherein the objective lens converges light irradiated on the recording medium at the position so that a focus of light irradiated on the recording medium can be adjusted on a recording face of the recording medium.
2. Description of the Related Art
In a disk reproducing apparatus for reading a signal recorded on a disk, which is a disk-shaped recording medium, when light is irradiated on the disk and the light reflected on the disk is converted into an electric signal, a focusing servo mechanism is provided in which a position of an objective lens (position of an objective lens with respect to a recording face of the disk), which converges light irradiated on the disk, in the optical axis direction is controlled so that a focus of light irradiated on the disk can follow the recording face of the disk.
In this case, focusing error signal FE, which shows a quantity of deviation from the focus of light irradiated on the disk to the recording face of the disk, changes as shown in FIG. 7 according to the position of the objective lens with respect to the recording face of the disk. When the objective lens is located at a position where the focus of light irradiated on the disk is adjusted on the recording face of the disk (This position will be referred to as an “in-focus-position” hereinafter.), focusing error signal FE is 0. When the objective lens is located in the vicinity of the in-focus-position, the more distant the objective lens is located from the disk, the more an absolute value of focusing error signal FE increases at the positive polarity. On the other hand, the closer the objective lens is located to the disk, the more an absolute value of focusing error signal FE increases at the negative polarity. However, when the objective lens is not located in the vicinity of the in-focus-position, the more distant the objective lens is located with respect to the disk, the more the absolute value decreases. Alternatively, the closer the objective lens is located with respect to the disk, the more the absolute value decreases.
In this case, the focusing servo mechanism controls a position of the objective lens with respect to the recording face of the disk so that focusing error signal FE becomes 0. Accordingly, in order to make the focusing servo mechanism function so that the focus of light irradiated on the disk can follow the recording face of the disk, it is necessary to activate the focusing servo mechanism in the vicinity of the in-focus-position.
Therefore, it is common to adopt the following method of activating the focusing servo mechanism. Operation for finding the in-focus-position, which will be referred to as “focus search” hereinafter, is conducted, and at the point of time when the in-focus-position is detected by this focus search (at point P in the case shown in FIG. 7), the focusing servo mechanism is activated.
The most fundamental operation conducted in the case of activating the focusing servo mechanism is shown in the flow chart of FIG. 8. First, when the entire pickup, which is a unit to irradiate light to the recording face of the disk, is moved in the direction perpendicular to the recording face of the disk, the objective lens provided in the pickup is made to come near or move away with respect to the recording face of the disk (S101). When focusing error signal FE comes to the zero-cross point (Yes at S102), it is judged that the objective lens coincides with the in-focus-position, and the focusing servo mechanism is activated (S103).
In this case, since the objective lens is supported in the optical axis direction by an elastic member such as a spring provided in the pickup, when the pickup is moved in a direction perpendicular to the recording face of the disk, damped oscillation of the natural frequency is generated on the objective lens. Therefore, for example, when the objective lens is made to come near the recording face of the disk by moving the pickup in a state in which the objective lens is completely stopped in the optical axis direction, a position of the objective lens with respect to the recording face of the disk changes as shown in FIG. 9. In other words, it is impossible to make the objective lens come near the recording face of the disk at a constant speed.
In the conventional focus search method, the pickup is moved without giving consideration to the above fact. Therefore, even when an operator intends to make the objective lens come near the recording face of the disk (even when an operator intends to make the objective lens move away from the recording face of the disk), the objective lens is actually moved away from the recording face (the objective lens is actually made to come near the recording face), by oscillation generated on the objective lens when the pickup is moved in the above way and deflection of recording face which are accumulated.
When the above phenomenon is caused, for example, in the case of focus search in which the pickup is moved so that the objective lens is made to come near the recording face of the disk, a wave-form of focusing error signal FE is formed into a shape shown in FIG. 10 in some cases. In FIG. 10, the objective lens comes near the recording face of the disk until it comes to point A, however, the objective lens is moved away from the recording face of the disk after it passes through point A. When the objective lens is moved too far away from the recording face of the disk, focusing error signal FE becomes 0 at point P′. Therefore, the in-focus-position is erroneously detected.
In this connection, according to the technique disclosed in JP-A-10-241169, in case where the tracking servo mechanism is activated after the pickup is moved in the radial direction of the disk, the tracking servo mechanism is not activated after the natural resonance oscillation caused in the movable section of the pickup is stabilized but the tracking servo mechanism is activated immediately before the rate of natural resonance oscillation becomes 0, so that the search time can be reduced. Therefore, the above problems cannot be solved by this technique.
According to the technique disclosed in JP-A-2001-134958, skipping of the track and deviating from the focus can be prevented by absorbing components included in the tracking error signal and the focusing error signal caused according to the rotary oscillation of the disk. Therefore, the above problems cannot be solved by this technique.